Connected: An Internet Encyclopedia
RFC 1812 - E.3.4 The Integrated IS-IS Algorithm

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### E.3.4 The Integrated IS-IS Algorithm

Integrated IS-IS uses an algorithm that is similar to but not quite identical to the OSPF Algorithm. Integrated IS-IS uses a different set of route classes, and differs slightly in its handling of type of service. The algorithm is:

1. Basic Match
2. IS-IS Route Classes
3. Longest Match
4. Weak TOS
5. Best Metric
6. Policy

Although Integrated IS-IS uses Weak TOS, the protocol is only capable of carrying routes for a small specific subset of the possible values for the TOS field in the IP header. Packets containing other values in the TOS field are routed using the default TOS.

Type of service support is optional; if disabled, the fourth step would be omitted. As in OSPF, the specification does not include the Policy step.

This algorithm has some advantages over the Revised Classic Algorithm:

1. It supports type of service routing.
2. Its rules are written down, rather than merely being a part of the Internet folklore.
3. It (obviously) works with Integrated IS-IS.

However, this algorithm also retains some of the disadvantages of the Revised Classic Algorithm:

1. Path properties other than type of service (e.g., MTU) are ignored.
2. As in the Revised Classic Algorithm, the details (or even the existence) of the Policy step are left to the discretion of the implementor.
3. It doesn't work with OSPF because of the differences between IS- IS route classes and OSPF route classes. Also, because IS-IS supports only a subset of the possible TOS values, some obvious implementations of the Integrated IS-IS algorithm would not support OSPF's interpretation of TOS.

The Integrated IS-IS Algorithm also has a further disadvantage (which is not shared by the Revised Classic Algorithm): IS-IS internal (intra-area or inter-area) routes are always considered to be superior to routes learned from other routing protocols, even in cases where the IS-IS route matches fewer bits of the destination address and doesn't provide the requested type of service. This is a policy decision that may not be appropriate in all cases.

Finally, it is worth noting that the Integrated IS-IS Algorithm's TOS support suffers from the same deficiency noted for the OSPF Algorithm.

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Connected: An Internet Encyclopedia
RFC 1812 - E.3.4 The Integrated IS-IS Algorithm